How long between initial interview and assesment center.

Crow

So i finally applied to join the army as a metalsmith (im a welder currently)
I started my application but held of finishing till i got my fitness in check and got my GCSE certificates sorted (lost them years ago).
But i got a call from my local career center to come in for a chat, i thought this was just a chat but i take it its actually an initial interview.

What i want to know is, what is the approx time frame from the initial interview to the assessment center? Ive been powerlifting/strongman training the last few years so would need a few months to shift some of the weight off and get my cardio in check.